For what its worth, comparison sake:

2010 2500 4x4 Big Horn Hemi, 3.73's, stock tires. Only mods are a PCV catch can and a Yukon Spin Free hub kit (test kit).

Towing my 28' extra height enclosed trailer from Seattle to Las Vegas and back (2266 miles round trip), 65mph cruise speed. I got 8.9mpg. Truck and trailer combined is just over 18,000 pounds rolling down the road.

That is over a number of mountain passes and bucking 30mph head winds for a couple hours of the trip.

Who said you need a Diesel...